U.S. patent application Ser. No. 10/115,825, entitled DUAL SPIN VALVE SENSOR WITH A LONGITUDINAL BIAS STACK, was filed on the same day, owned by a common assignee and having the same inventors as the present invention.
1. Field of the Invention
This invention relates in general to magnetic transducers for reading information signals from a magnetic medium and, in particular, to a dual magnetic tunnel junction sensor with a longitudinal bias stack between first and second magnetic tunnel junction structures of the dual sensor.
2. Description of the Related Art
Computers often include auxiliary memory storage devices having media on which data can be written and from which data can be read for later use. A direct access storage device (disk drive) incorporating rotating magnetic disks is commonly used for storing data in magnetic form on the disk surfaces. Data is recorded on concentric, radially spaced tracks on the disk surfaces. Magnetic heads including read sensors are then used to read data from the tracks on the disk surfaces.
In high capacity disk drives, magnetoresistive (MR) read sensors, commonly referred to as MR sensors, are the prevailing read sensors because of their capability to read data from a surface of a disk at greater track and linear densities than thin film inductive heads. An MR sensor detects a magnetic field through the change in the resistance of its MR sensing layer (also referred to as an xe2x80x9cMR elementxe2x80x9d) as a function of the strength and direction of the magnetic flux being sensed by the MR layer.
The conventional MR sensor operates on the basis of the anisotropic magnetoresistive (AMR) effect in which an MR element resistance varies as the square of the cosine of the angle between the magnetization in the MR element and the direction of sense current flowing through the MR element. Recorded data can be read from a magnetic medium because the external magnetic field from the recorded magnetic medium (the signal field) causes a change in the direction of magnetization in the MR element, which in turn causes a change in resistance in the MR element and a corresponding change in the sensed current or voltage.
Another type of MR sensor is the giant magnetoresistance (GMR) sensor manifesting the GMR effect. In GMR sensors, the resistance of the MR sensing layer varies as a function of the spin-dependent transmission of the conduction electrons between magnetic layers separated by a nonmagnetic spacer layer and the accompanying spin-dependent scattering which takes place at the interface of the magnetic and nonmagnetic layers and within the magnetic layers.
GMR sensors using only two layers of ferromagnetic material (e.g., Nixe2x80x94Fe) separated by a layer of nonmagnetic material (e.g., copper) are generally referred to as spin valve (SV) sensors manifesting the SV effect.
FIG. 1 shows a prior art SV sensor 100 comprising end regions 104 and 106 separated by a central region 102. A first ferromagnetic layer, referred to as a pinned (or reference) layer 120, has its magnetization typically fixed (pinned) by exchange coupling with an antiferromagnetic (AFM) layer 125. The magnetization of a second ferromagnetic layer, referred to as a free (or sense) layer 110, is not fixed and is free to rotate in response to the magnetic field from the recorded magnetic medium (the signal field). The free layer 110 is separated from the pinned layer 120 by a nonmagnetic, electrically conducting spacer layer 115. Hard bias layers 130 and 135 formed in the end regions 104 and 106, respectively, provide longitudinal bias for the free layer 110. Leads 140 and 145 formed on hard bias layers 130 and 135, respectively, provide electrical connections for sensing the resistance of SV sensor 100. In the SV sensor 100, because the sense current flow between the leads 140 and 145 is in the plane of the SV sensor layers, the sensor is known as a current-in-plane (CIP) SV sensor. IBM""s U.S. Pat. No. 5,206,590 granted to Dieny et al., incorporated herein by reference, discloses a SV sensor operating on the basis of the GMR effect.
Another type of spin valve sensor is an antiparallel (AP)-pinned SV sensor. The AP-pinned SV sensor differs from the simple spin valve sensor in that an AP-pinned structure has multiple thin film layers instead of a single pinned layer. The AP-pinned structure has an antiparallel coupling (APC) layer sandwiched between first and second ferromagnetic pinned layers. The first pinned layer has its magnetization oriented in a first direction by exchange coupling to the antiferromagnetic (AFM) pinning layer. The second pinned layer is immediately adjacent to the free layer and is antiparallel exchange coupled to the first pinned layer because of the minimal thickness (in the order of 8 xc3x85) of the APC layer between the first and second pinned layers. Accordingly, the magnetization of the second pinned layer is oriented in a second direction that is antiparallel to the direction of the magnetization of the first pinned layer.
The AP-pinned structure is preferred over the single pinned layer because the magnetizations of the first and second pinned layers of the AP-pinned structure subtractively combine to provide a net magnetization that is much less than the magnetization of the single pinned layer. The direction of the net magnetization is determined by the thicker of the first and second pinned layers. A reduced net magnetization equates to a reduced demagnetization field from the AP-pinned structure. Since the antiferromagnetic exchange coupling is inversely proportional to the net magnetization, this increases exchange coupling between the first pinned layer and the antiferromagnetic pinning layer. The AP-pinned spin valve sensor is described in commonly assigned U.S. Pat. No. 5,465,185 to Heim and Parkin which is incorporated by reference herein.
Another type of magnetic device currently under development is a magnetic tunnel junction (MTJ) device. The MTJ device has potential applications as a memory cell and as a magnetic field sensor. The MTJ device comprises two ferromagnetic layers separated by a thin, electrically insulating, tunnel barrier layer. The tunnel barrier layer is sufficiently thin that quantum-mechanical tunneling of charge carriers occurs between the ferromagnetic layers. The tunneling process is electron spin dependent, which means that the tunneling current across the junction depends on the spin-dependent electronic properties of the ferromagnetic materials and is a function of the relative orientation of the magnetizations of the two ferromagnetic layers. In the MTJ sensor, one ferromagnetic layer has its magnetization fixed, or pinned, and the other ferromagnetic layer has its magnetization free to rotate in response to an external magnetic field from the recording medium (the signal field). When an electric potential is applied between the two ferromagnetic layers, the sensor resistance is a function of the tunneling current across the insulating layer between the ferromagnetic layers. Since the tunneling current that flows perpendicularly through the tunnel barrier layer depends on the relative magnetization directions of the two ferromagnetic layers, recorded data can be read from a magnetic medium because the signal field causes a change of direction of magnetization of the free layer, which in turn causes a change in resistance of the MTJ sensor and a corresponding change in the sensed current or voltage. IBM""s U.S. Pat. No. 5,650,958 granted to Gallagher et al., incorporated in its entirety herein by reference, discloses an MTJ sensor operating on the basis of the magnetic tunnel junction effect.
FIG. 2 shows a prior art MTJ sensor 200 comprising a first electrode 204, a second electrode 202, and a tunnel barrier layer 215. The first electrode 204 comprises a pinned layer (pinned ferromagnetic layer) 220, an antiferromagnetic (AFM) layer 230, and a seed layer 240. The magnetization of the pinned layer 220 is fixed through exchange coupling with the AFM layer 230. The second electrode 202 comprises a free layer (free ferromagnetic layer) 210 and a cap layer 205. The free layer 210 is separated from the pinned layer 220 by a nonmagnetic, electrically insulating tunnel barrier layer 215. In the absence of an external magnetic field, the free layer 210 has its magnetization oriented in the direction shown by arrow 212, that is, generally perpendicular to the magnetization direction of the pinned layer 220 shown by arrow 222 (tail of an arrow pointing into the plane of the paper). A first lead 260 and a second lead 265 formed in contact with first electrode 204 and second electrode 202, respectively, provide electrical connections for the flow of sensing current Is from a current source 270 to the MTJ sensor 200. Because the sensing current is perpendicular to the plane of the sensor layers, the MTJ sensor 200 is known as a current-perpendicular-to-plane (CPP) sensor. A signal detector 280, typically including a recording channel such as a partial-response maximum-likelihood (PRML) channel, connected to the first and second leads 260 and 265 senses the change in resistance due to changes induced in the free layer 210 by the external magnetic field.
Two types of current-perpendicular-to-plane (CPP) sensors have been extensively explored for magnetic recording at ultrahigh densities (xe2x89xa720 Gb/in2). One is a GMR spin valve sensor and the other is a MTJ sensor. Two challenging issues are encountered when the CPP sensor is used for ever increasing magnetic recording densities. First, the GMR coefficient may not be high enough to ensure adequate signal amplitude as the sensor width is decreased and second, magnetic stabilization of the sense layer can be difficult due to the use of insulating layers to avoid current shorting around the active region of the sensor. A dual CPP sensor can be used to provide increased magnetoresistive response to a signal field due to the additive response of the dual sensors. IBM""s U.S. Pat. No. 5,287,238 granted to Baumgart et al. discloses a dual CIP SV sensor. However, sensor stability still remains a major concern.
There is a continuing need to increase the MR coefficient and reduce the thickness of GMR sensors while improving sensor stability. An increase in the GMR coefficient and reduced sensor geometry equates to higher bit density (bits/square inch of the rotating magnetic disk) read by the read head.
It is an object of the present invention to disclose a dual magnetic tunnel junction (MTJ) sensor with improved sensor stabilization.
It is another object of the present invention to disclose a dual MTJ sensor having a longitudinal bias stack between a first MTJ stack and a second MTJ stack to provide improved stabilization of the sense (free) layers of the first and second MTJ stacks.
It is a further object of the present invention to disclose a dual MTJ sensor having a longitudinal bias stack comprising a first decoupling layer, a first ferromagnetic (FM1) layer, an antiferromagnetic (AFM) layer, a second ferromagnetic (FM2) layer and a second decoupling layer disposed between the sense layers of first and second MTJ stacks.
It is yet another object of the present invention to disclose a dual MTJ sensor having a longitudinal bias stack disposed between first and second MTJ stacks to provide three flux closures for improved sensor stability. A first flux closure provides stability of the first MTJ stack, a second flux closure provides stability of the second MTJ stack, and a third flux closure provides cancellation of demagnetizing fields from first and second antiparallel (AP)-pinned layers of the dual MTJ sensor.
In accordance with the principles of the present invention, there is disclosed a preferred embodiment of the present invention wherein a dual MTJ sensor comprises a first MTJ stack, a second MTJ stack and a longitudinal bias stack disposed between first and second sense layers of the dual MTJ sensor. The first MTJ stack comprises a first antiferromagnetic (AFM1) layer, a first AP-pinned layer, a first tunnel barrier layer and a first sense layer. The second MTJ stack comprises a second antiferromagnetic (AFM2) layer, a second AP-pinned layer, a second tunnel barrier layer and a second sense layer. The longitudinal bias stack comprises a third antiferromagnetic (AFM3) layer sandwiched between a first ferromagnetic (FM1) layer and a second ferromagnetic (FM2) layer, and first and second decoupling layers in laminar contact with the FM1 and FM2 layers, respectively.
The AFM1 and AFM2 layers are set by annealing the MTJ sensor at elevated temperature (about 280xc2x0 C.) in a large magnetic field (about 10,000 Oe) oriented in a transverse direction perpendicular to an air bearing surface (ABS) to orient the magnetizations of the first and second AP-pinned layers. The AFM3 layer, formed of antiferromagnetic material having a lower blocking temperature (temperature at which the pinning field reaches zero Oe) than AFM1 and AFM2, is set by the annealing but is reset by a second annealing step at a lower temperature (about 240xc2x0 C.) in a smaller magnetic field (about 200 Oe) oriented in a longitudinal direction parallel to the ABS to reorient the magnetizations of the FM1 and FM2 layers from the transverse to the longitudinal direction without reorienting magnetizations of the first and second AP-pinned layers. After the two annealing steps, the magnetizations of the first and second AP-pinned layers are oriented perpendicular to the ABS with net magnetic moments canceling each other, and the FM1 and FM2 layers are oriented in the longitudinal direction. The magnetization of the FM1 layer forms a flux closure with the magnetization of the first sense layer and the magnetization of the FM2 layer forms a flux closure with the magnetization of the second sense layer. The first and second sense layers can be stabilized through magnetostatic interactions induced from the first and second flux closures, respectively.
The above as well as additional objects, features, and advantages of the present invention will become apparent in the following detailed description.